It has long been recognized that shouting through closed doors is an unsatisfactory way for persons in apartments, hotel rooms, vacation condominiums, etc., to communicate with attempted visitors in order to be sure that the attempted visitors are not impostors or criminals. The door muffles the sound, and makes it difficult for the apartment dwellers to recognize voices with accuracy.
Many apartment (for example) doors do not have peep holes, and it is not authorized for holes to be drilled in the doors. The same applies to hotel room doors, where the danger of unwanted intruders is perhaps even greater than relative to apartments.
It has been proposed several times to mount outside and inside intercom units on the doors of dwellings, using the peep holes for the mounting and connecting means. Such proposals are of no value to persons who wish to speak through doors of apartments, hotel rooms, condominiums, etc., where there are no peep holes. Furthermore, and very importantly, such apparatus is not readily portable, nor is it quickly mountable and demountable.